Big girls certainly can get pregnant! They might just need a feisty, smaller male.
I have a big girl, a beautiful beige, who is a super slow breeder. She has had one litter for me since 2010. After she and her previous male proved they were just going to hang out, I swapped in a smaller but otherwise gorgeous proven standard male and he bred her as soon as she was in heat.
Keep the faith! And don't be afraid to swap mates.